Conventionally, there is commonly used a battery pack which obtains a required voltage and capacity by connecting a plurality of cells in series or parallel. In addition, in recent years, in terms of reductions in consumption of fossil fuel and emission of CO2, as a power supply for driving a motor of an electric car or a hybrid car, there are growing expectations for the battery pack.
Such battery pack for the electric vehicle requires a voltage and a capacity larger than those for the battery pack for domestic use. Accordingly, the battery pack has been constituted mainly by connecting a large number of dedicated large-capacity cells in series. However, recently, there is started the adoption of a technology capable of coping with various uses through modularization by connecting a plurality of general-purpose batteries used for domestic use in series or parallel.
The modularization technology has an advantage that flexibility in mounting the battery pack in a limited space in the vehicle is improved, and the use of the general-purpose battery allows a significant reduction in cost due to a mass production effect. In addition, a relatively large number of batteries are connected in parallel and used, and hence, even when an abnormality occurs in some of the batteries, the supply of power can be continued for a certain time period. Consequently, it is possible to avoid a system failure.
However, in the case where the supply of power is continued even when the abnormality occurs in some of the batteries, it becomes important to secure safety of not only the individual batteries but also the entire battery module as the assembly of the individual batteries. That is, when an excessive current flows in the battery due to, e.g., an internal short-circuit, unusual heat generation occurs, an internal pressure is increased, and high-temperature gas gushes out so that the surrounding area is thereby adversely affected and the battery is degraded in a chain reaction manner.
There is proposed a method for exhausting smoke to the outside of a cabin by automatically starting an air conditioner or opening a window in advance to communicate the cabin with the outside of a vehicle when a vehicle collision or the like is anticipated such that, even when an impact is applied to a battery pack and the smoke (gas) generated from an internal lithium ion battery enters into the cabin, the smoke can be exhausted to the outside of the vehicle (see, e.g., Patent Document 1).
In addition, there is proposed a method for monitoring a state when gas is exhausted from a battery by using a gas sensor and a gas temperature sensor provided inside a gas exhaust path (see, e.g., Patent Document 2).
In the conventional art described in Patent Document 1, a collision detection sensor or a radar device provided in the vehicle predicts the vehicle collision, and the air conditioner is started or the window is opened. Consequently, even when the vehicle collision does not actually happen and the impact is not applied to the vehicle, there are cases where the air conditioner is started or the window is opened. Such operation may be an operation against the intention of a user and may give an uncomfortable feeling to the the user. There is a possibility that the smoke occurs depending on the state of the lithium ion battery even when the impact is not applied to the battery pack, and the conventional art of Patent Document 1 has had a problem that the abnormality of the battery cannot be detected in a case where the smoke occurs due to causes other than the impact.
In the conventional art described in Patent Document 2, when the lithium ion battery generates the gas, it is possible to detect the generation of the gas by using the gas sensor and the gas temperature sensor and report the occurrence of the abnormality to the user driving the vehicle by, e.g., turning on an alarm light provided in an instrument panel of the vehicle. However, in many cases, the generation of the gas from the battery continues for a relatively short time period, e.g., several seconds. Accordingly, even when the gas is generated from the battery, the alarm light is turned on only for several seconds, and hence the user may overlook the turning-on of the alarm light. Further, in a case where the abnormality occurs in the battery and the gas is generated when the vehicle is not driven, especially when the power supply of the vehicle is turned off, a control circuit for detecting the gas itself is not operated, and hence the generation of the gas cannot be detected. Thereafter, when the power supply of the vehicle is turned on, the generation of the gas is already stopped, and hence there has been a problem that the abnormality of the battery cannot be detected after all.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-099075
Patent Document 2: WO 2009/150965